The present invention relates to coatings that inhibit the formation and adhesion of deposits on surfaces that contact hydrocarbon fluids, such as hydrocarbon fluid containment systems including but not limited to gas turbine engines, furnaces used to produce polymers, diesel engines, etc. More particularly, this invention relates to a coating system for preventing or reducing the deposition of carbonaceous deposits on the surfaces of fuel nozzles, swirlers, oil scavenge lines, and other fuel and lubrication system components of gas turbine engines, as well as a process and apparatus for depositing the coating system.
Thermal instability, or in the case of fuels, fuel instability, generally refers to the formation of undesired carbonaceous deposits that occurs when hydrocarbon fluids, such as fuels and lubricating oils, are at elevated temperatures. In the case of fuels, it is generally accepted that there are two distinct mechanisms occurring within two overlapping temperature ranges. In the first mechanism, referred to as the coking process, a generally consistent increase in the rate of formation of carbonaceous coke deposits occurs above temperatures of about 650° F. (about 345° C.). Coke formation is the result of high levels of hydrocarbon pyrolysis, and eventually limits the usefulness of the fuel. A second mechanism primarily occurs at lower temperatures, generally in the range of about 220° F. to about 650° F. (about 105° C. to about 345° C.), and involves oxidation reactions that lead to polymerization and carbonaceous gum deposits.
In the past, the solution to the problem of gum and coke formation was primarily directed toward placing limitations on fuel chemistry and impurities associated with fuels, as disclosed in U.S. Pat. Nos. 2,698,512, 2,959,915 and 3,173,247. However, the propensity for gum and coke formation is increased with certain hydrocarbon fluids for fuels, oils, lubricants, petrochemical processes (plastics and synthetics) and the like, especially those derived from nonpetroleum sources, such as shale and coal, which can exhibit significantly more problems with thermal instability because of their high content of olefins, sulfur and other compounds. The consequences of thermal instability and fuel instability are of even greater significance with developing technology that requires processes and machinery to operate at higher temperatures, as afforded by advances in materials technology. Accordingly, fluid containment articles that are resistant to or prevent the formation of adverse decomposition products and foulants are necessary in applications where thermal instability, including fuel instability, is a problem as a result of exposure of such fluids to high temperatures. Particularly notable applications include the fuel-handling and lubrication system components of gas turbine engines, which includes the fuel nozzles and swirlers that mix fuel and air before injecting the mixture into the combustor, oil scavenge lines that transport lubrication oil to critical bearings of the engine hot section, and bearing housings and seal runners that house the bearings requiring lubrication. With the advent of higher engine operation temperatures and the use of fuel as a heat sink, there is an increased likelihood that carbonaceous deposits can severely choke the flow of fuel and air through fuel nozzles and swirlers, affecting operating conditions (e.g., mixing of fuel and air, proper flow of fuel and oxygen into the combustor) and may reduce fuel efficiency and increase emissions. Higher engine operation temperatures also increase the likelihood that carbonaceous deposits will choke the flow of lubrication oil through scavenge lines, leading to loss of lubrication to critical bearings and flooding of the oil sump.
It has been recognized that deposits can form as a result of a reaction between a hydrocarbon fluid and its containment wall. In U.S. Pat. No. 4,078,604, heat exchangers are provided with thin-walled corrosion-resistant layers of electrodeposited gold or similar corrosion-resistant metals on the walls of the cooling channels in order to make the surfaces corrosion resistant to such storable liquid fuels as red fuming nitric acid. In this case, the wall is protected from corrosion, and the intent is not to prevent deposit formations. Furthermore, gold readily diffuses into other materials at elevated temperatures, and therefore is unsuitable as a protective coating for high temperature applications, e.g., temperatures associated with gum and coke formation.
More recently, coating systems specifically directed to inhibiting the formation and adhesion of carbonaceous deposits have been taught. For example, U.S. Pat. Nos. 5,805,973, 5,891,584, 5,923,944, and 6,156,439, all assigned to the assignee of the present invention and incorporated herein by reference, teach the use of coke barrier coatings (CBC's) that eliminate or modify the surface reactions which lead to formation of thermal instability deposits from hydrocarbon fluids, and reduce adhesion of such deposits. These patents are generally directed to ceramic coatings that are especially capable of reducing deposits at very high temperatures, e.g., above 650° F. (about 345° C.). As an example, U.S. Pat. Nos. 5,805,973 and 5,891,584 disclose coatings that catalyze thermal decomposition in the hydrocarbon fluid to actually promote the formation of coke, which is substantially nonadherent to the coatings.
Many applications exist where there is a particular need for coatings that can significantly reduce the formation and adhesion of carbonaceous deposits at lower temperatures, such as the above-noted 105 to 345° C. range for gum deposits. For this type of hardware, reductions in hydrocarbon deposits have been achieved with the use of coatings that are not reactive with hydrocarbons. In situations where heat transfer from the containment walls is a major contributor to the fluid temperature, thermally-reflective (low emissivity) coatings that reduce heat transfer to the hydrocarbon fluid have been employed to reduce deposit formation. Notably, the CBC systems taught by U.S. Pat. Nos. 5,805,973, 5,891,584, 5,923,944, and 6,156,439 do not have the correct optical properties, including low emissivity, to function as radiation shields. While CBC systems of the prior art can be combined with low-emissivity coatings, a significant drawback is the additional volume, weight and cost incurred.
Certain applications also exist where sustained temperatures result in the formation and adhesion of both coke and gum deposits, a particular example being fuel nozzles and swirlers whose sustained surface temperatures can be in the range of, for example, about 600 to 800° F. (about 315 to 425° C.). For such applications, it would be desirable if an improved coating system were available that was capable of reducing the formation and adhesion of both carbonaceous coke and gum deposits.